The cnidarian phylum of animals—comprised of jellyfish, tube-dwelling anemones, minute polyps, and certain types of coral—have a rather unique adaptation that allows them to thrive against much larger and cleverer creatures. All of the organisms in this phylum are characterized by specialized cells called cnidocytes, not found elsewhere in the animal kingdom.
Structures within the cnidocyte cells function as projectiles that are essential for the survival of these creatures. Cnidocytes are useful both for ensnaring prey and for defending these unusual creatures against other marine animals that might otherwise view them as a delicacy.
Cnidocyte Cell Structure and Function
Each cnidocyte is contained within a tough capsule known as a cnida, which stores high concentrations of calcium ions. Each cnida has an opening covered by a lid called an operculum—configured as either a single hinged flap, or three pie-shaped flaps. A coiled tubule resides inside the capsule until it is triggered by the cnidocil, a hair-like sensory device activated by a combination of physical contact and chemical response.
When the cell is activated, calcium ions are released into the fluid in the cell, forcing a rapid influx of water. This triggers the coiled tubule inside to spring out- a process that can take as little as seven hundred nanoseconds to complete. Cnidocytes can only be activated once then it takes approximately two days for the cnidarian to grow new stinging cells.
Some varieties of nudibranchs and other sea slugs store toxic cnidocytes on the tips of their body projections from previously digested jellyfish
Cnidocytes can be separated into three classes. The most prevalent of these is the nematocyst, a hollow tubule with spikes on the end that were developed to penetrate the skin of their quarry. Most cnidarian species have nematocysts, but many of them are too small to puncture human skin. Nematocysts often contain toxins—most commonly neurotoxins—and can be quite painful when they do puncture the skin.
Spirocysts, long, hollow tubules that are adhesive in nature, are found on several varieties of sea anemone, as well as on stony and soft corals. These stationary carnivores use spirocysts to entangle and capture their prey.
Ptychocysts are a class of cell unique to tube anemones. Tube anemones, also referred to as burrowing anemones, are able to burrow into the ground and build protective tubes for themselves by shooting sticky threads from their ptychocysts.
Handling Jellyfish Stings
- Rinse the area with vinegar or warm water (avoid seawater or cool water)
- Avoid rubbing the area or scraping the stingers off as this can push additional toxins into the wound
- Pull off any remaining tentacles with tweezers, do not use your hands
Dangerous Symptoms: Call an Ambulance
- Stings cover a large portion of the body or are located on sensitive areas such as the eyes or mouth
- Difficulty breathing or swallowing
- Swollen lips or tongue
- Nausea or vomiting
- Muscle spasms
The type of neurotoxin that a cnidarian produces and the damage it can cause to the human body varies greatly between species, as does the overall size of the cnidocyte cell. Sea anemones are fearsome, deadly predators to marine animals like small fish, but their cells are so tiny that they aren’t capable of penetrating human skin.
Jellyfish stings, on the other hand, are more likely to be noticed by humans. Depending on the species of jellyfish and the amount of toxin injected, effects can range from a slight pain accompanied by mild redness, itching, or numbness, to cardiac arrest.